This is male subimago, formerly known as Stenonema. All but one species (not this one) are now Macaffertium. It can be distinguished from Stenacron by the grey eyes (Stenacron eyes are bright yellow) and the lack of black spots on crossveins in the bulla region of the wing.

Thanks, David, for your comments. Very helpful.

Thanks, Kerry, for your comments. Do you have specific references or texts you could recommend for these particular insects?

Lew, note that these are older publications, I'm sure many updated sources are available.

1981, Aquatic Entomology, by W.P.McCafferty.
A good general resource with references for more specific information.

1976, The Mayflies of North and Central America, by G.F.Edmunds, Jr., S.L.Jensen, and L.Berner.
The keys to nymphs and adults are well illustrated. Species are listed with references given for further research.

Many books can be found that were/are produced for the flyfishing crowd. The taxonomy is reasonable and the photographs are usually of high quality.

Now I'll have to go check out what's new, if I find anything good, you'll receive the bill if it gets excessive:).
KSM

Thanks, Kerry. I will look into these suggestions. LGS